Lysimeter study of explosives originated nitrogen and in situ sulphide oxidation

Teemu Karlsson, Geological Survey of Finland, P.O. BOX 1237, FI-70211 Kuopio, FINLAND, e-mail: teemu.karlsson(at)gtk.fi

Introduction

A long term study was conducted at a dimension stone quarry in eastern Finland to study the aging of the waste rock material and the amount of nitrogen contamination on the stone surfaces. Also some data about in situ sulphide oxidation was gathered. Two aging test units or lysimeters were installed at the quarry site.

The test units consisted of 1 m3 lysimeters, which were filled with left over rock material collected immediately after the excavation of a large diabase block. The rock material in both lysimeters was meant to be identical and the purpose of setting up two units was to examine the repeatability of the test. The Unit 1 contained approximately 1970 kg of rocks with 38 % porosity, and the Unit 2 contained approximately 1790 kg of rocks with 44 % porosity. The test units are presented in Figure 1.

Figure 1. Two lysimeters installed at a study site. The rainwater flowing through the units was collected in the green plastic tanks below. Photo: Lauri Solismaa / GTK

The rainwater flowing through the lysimeters was collected in separated 70-liters plastic tanks and analysed frequently, 9 times in total, between 10th of October 2012 and 16th of October 2013, excluding the winter months when the test units were frozen. The interval of samplings was 2 weeks during the first month and once in a month during the rest of the monitoring period.
The monitored parameters included nitrogen species (NO2, NO3, NH4 and total-N), Cl–, SO42- pH, EC and temperature. The chloride (Cl–) is present in the rock dust and in the blasting residuals as sodium chloride  (Forcit Oy 2013), and is considered to be a good indicator of the first flush of water through the test units as the amount of Cl– in left over stone does not increase by an external source. The sulphate (SO42-) is mainly derived from sulphide oxidation, first during blasting of rock, and later from in situ sulphide mineral oxidation (Bailey et al. 2013).

Results and discussion

The monitored concentrations and values followed similar trends in both test units. At first the electric conductivity and concentrations, e.g. of total-N and Cl– were relatively high, but declined rapidly to the natural background values after the “first flush”. Cl– declined more sharply and remained low for the remainder of the study. The nitrogen concentrations were more erratic, but the absence of Cl– suggests that most of the nitrogen was delivered from rainwater / atmosphere. The nitrogen and Cl–concentrations are presented in Figure 2.
Figure 2. Total nitrogen and chloride concentrations in the recharge waters of the Test Units 1 and 2 and total-N of rainwater in Maaninka (monthly averages during the years 2004-2012) measured by the Finnish Environment Institute (Environmental Administration of Finland 2014).
In the beginning of the observation period the coincident rise of SO4 with other blasting agents suggest that initial release of SO4 was mainly derived from sulphide oxidation during blasting. After the first weeks the absence of Cl– and erratic amounts of SO4 suggest that most of the SO4 has been derived from in situ sulphide mineral oxidation within the test units. The rock material of the test units contained about 0.3 % pyrite, which was a relatively small amount but appreciably impacted pH, which dropped around 7. A correlation between the pH values and the amount of SO4 was observed; increase in SO4 decreased the pH. Some anomalies were observed, especially around June 2013, when the SO4 concentrations were low, but also pH remained more acidic. This may be caused by measurement errors or secondary mineral precipitation processes in the test units. The SO4 concentrations and pH values are presented in Figure 3.
Figure 3. Sulphate (SO4) concentrations and pH values in the recharge waters of the Test Units 1 and 2.
In the end of the observation period, the test units were filled (washed) with tap water and sampled three times to remove and detect the remaining explosives-originated nitrogen and to determine the porosity and approximately the weight of the rocks inside the test units.
The amounts of released total nitrogen from the lysimeters were calculated by multiplying the amounts of water (litres) by the concentrations of total nitrogen. The nitrogen concentrations of the first flush were determined by subtracting the background value (0.6 mg/L in October) from the measured concentrations. The first flush includes the first samplings in October 2012 when the concentrations are higher than the background values of rainwater.
According to the flushing of the test units after the observation period, about half of the total nitrogen seemed to be left in the test units after one year of ageing. The amount of detected explosives-originated nitrogen in the Test Unit 1 was 138.8 mg i.e. 0.07 mg/kg and in the Test Unit 2 32.5 mg i.e. 0.02 mg/kg. Presumably some traces of explosives-originated nitrogen was released also after the first flush, but the nitrogen concentrations were so close to the background values that assessment of exact numbers is difficult. Also some small amounts were probably left in the stone material after the final washings. Considering the uncertainty factors, the scale of the explosives-originated nitrogen amount in the type of stone material used in this study seems to be around 0.1 mg/kg or 0.1 g/t.
The rock material in the test units presented a ”Worst case”; real waste rock material contains larger amount of big boulders and thus smaller surface area / kg (= less explosives-N / kg) than the finer material in the test units.

Conclusions of the lysimeter study

Based on the observation period of more than a year the “first flush” of recharge water occurs shortly, only after few weeks, after which the nitrogen drainage settles to natural rainwater levels. Based on the calculations the first flush removes approximately half of the total nitrogen.
Chloride, which is present in the rock dust and in the pipe charge explosives as sodium chloride, was observed to be a good indicator and representative of the first flush of recharge water, as the amount of Cl– in leftover stone does not increase due to an external source. Nitrogen, on the other hand, had a more erratic tail, as natural rainwater contains significant amounts of nitrogen species. Sulphide in situ oxidation was also detected inside the test units.
Calculations on the nitrogen leached out of the test units during the observation period and the remaining nitrogen detected on the rocks after the test period indicate that some explosive residuals remain within the test material. The remaining nitrogen is leached out fairly slowly, promoting also loss to the atmosphere.
The two seemingly identical units had different nitrogen concentrations, which suggest that the undetonated explosives are heterogeneously distributed in a waste rock pile and that not all of the leftover stones have been in contact with explosives.
Leftover stones produced by natural stone industry contain less explosives-originated nitrogen than the waste rocks produced by larger mining activities. The total amount of explosives-originated nitrogen on the waste rock material of a natural stone quarry seems to be on the scale of 0.1 mg/kg, which is well below the amount detected on the waste rock produced by a larger scale mines, e.g. 4.mg/kg at the Darvik diamond mine (Bailey et al. 2013).

Acknowledgements

This study was conducted as a part of the Green Mining project MINIMAN, which concentrated on the behaviour of nitrogen compounds in mining environments, and developing technologies for management of nitrogen discharges. The research project was realized during the years 2012-2014 as a cooperative project between Geological Survey of Finland (GTK), Technical Research Centre of Finland (VTT) and Tampere University of Technology (TTY) together with several industrial and international partners.
The study has been published in the Proceedings of the 10th International Conference on Acid Drainage & IMWA Annual Conference (Karlsson, T. and Kauppila, T. 2015. Release of Explosives Originated Nitrogen from the Waste Rocks of a Dimension Stone Quarry. In: Brown, A., Bucknam, C., Carballo, M., Castendyk, D., Figueroa, L, Kirk, L., McLemore, V., McPhee, J., o´Kane, M., Seal, R., Wiertz, J., Williams, D., Wilson, W., Wolkersdorfer, C. (Eds): 10th International Conference on Acid Rock Drainage & IMWA Annual Conference. pp. 1-10.)

Evaluation of the method

Field scale lysimeters serve as a valuable link in the interpretation of laboratory and field environmental data for leaching of harmful substances. They give more accurate results related to laboratory tests as the size of the sample is larger and they are installed in actual field conditions, but they are more expensive and time consuming. Field scale experiments, like the one described in this study, are suitable for nitrogen investigations as the undetonated explosives leach out relatively fast. For ARD prediction and metal leaching assessments this kind of tests should be implemented at early stages of the mining operations, since they require a number of years before meaningful results and conclusions can be made (Urrutia et al. 2011).
As waste rock material is usually relatively heterogenous, various lysimeters should be installed to obtain more precise data. Also the size of the lysimeter is essential; the larger the lysimeter, more accurate information will be obtained.

References

Bailey, B., Smith, L.J.D., Blowes, D., Ptacek, C., Smith, L. & Sego, D. 2013 The Diavik Waste Rock Project: Persistence of contaminants from blasting agents in waste rock effluent. Applied Geochemistry 36: 256-270
Forcit Oy 2013. K-pipecharge / KK-pipecharge Product information version 02.04.2013.
Environmental Administration of Finland 2014. HERTTA Database version 5.6.
Urrutia, P., Wilson, W., Aranda, C., Peterson, H., Blackmore, S., Sifuentes, F. & Sanchez, M. 2011. Design and Construction of Field-Scale Lysimeters for the Evaluation of Cover Systems at the Antamina Mine, Peru. Proceedings Tailings and Mine Waste 2011, Vancouver, BC, November 6 to 9, 2011.